Some known systems for purifying exhaust gases emitted from internal combustion engines have an exhaust gas purifier disposed in the exhaust passage, which may comprise a hydrocarbon adsorbent such as zeolite or a hydrocarbon adsorbing catalyst comprising a composite combination of a hydrocarbon adsorbent and a three-way catalyst, for adsorbing hydrocarbons (HC) in the exhaust gas while the catalytic converter such as a three-way catalyst or the like is not functioning sufficiently, i.e., while the catalytic converter is not sufficiently activated as when the internal combustion engine starts to operate at a low temperature. The hydrocarbon adsorbent has a function to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100° C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250° C., for example.
The applicant of the present application has proposed the following technique of monitoring a state, e.g., a deteriorated state, of a hydrocarbon adsorbent of the type described above: The inventors of the present invention have found that the hydrocarbon adsorbent is capable of adsorbing not only hydrocarbons contained in exhaust gases, but also moisture contained in exhaust gases. The ability of the hydrocarbon adsorbent to adsorb moisture, i.e., the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent, is highly correlated to the ability of the hydrocarbon adsorbent to adsorb hydrocarbons, i.e., the maximum amount of hydrocarbons that can be adsorbed by the hydrocarbon adsorbent. As the hydrocarbon adsorbent progressively deteriorates, both the ability to adsorb moisture and the ability to adsorb hydrocarbons are progressively lowered in the same manner. Therefore, when the ability of the hydrocarbon adsorbent to adsorb moisture is evaluated, the ability of the hydrocarbon adsorbent to adsorb hydrocarbons can also be evaluated based on the evaluated ability to adsorb moisture. According to the technique proposed by the applicant, a humidity sensor is disposed downstream of the hydrocarbon adsorbent. Using output data of the humidity sensor, the ability of the hydrocarbon adsorbent to adsorb moisture and hence the ability of the hydrocarbon adsorbent to adsorb hydrocarbons are grasped, thus monitoring the deteriorated state of the hydrocarbon adsorbent.
For adequately evaluating the deteriorated state of the hydrocarbon adsorbent using output data from the humidity sensor in various many environments that the internal combustion engine is subject to while in operation, the humidity sensor that is exposed to high-temperature exhaust gases is required to be highly durable over a long period of time, and also required to have minimum changes in the aging characteristics thereof and to suffer minimum characteristic variations among individual units of humidity sensors.
However, it is generally difficult for humidity sensors to fully satisfy all the above requirements. It would need a large expenditure of money and labor to develop humidity sensors that fully satisfy all the above requirements, and such humidity sensors would be highly expensive to manufacture.
The present invention has been made in view of the above background. It is an object of the present invention to provide a hydrocarbon adsorbent state monitoring apparatus for adequately monitoring the deteriorated state or the like of a hydrocarbon adsorbent by compensating for effects of changes in the characteristics of humidity sensors that are used and also compensating for effects of characteristic variations among individual units of the humidity sensors.